As for those who believe that only aikido will help aikido, you may want to consider that the number of micro-tears (musculo-tendonal or other) which occur with training can be reduced if the body is given a rest, yet one can rest and still train if there is variety in training.

A very good point.

I don't think anybody said only Aikido will help Aikido but more along the lines of the best thing for Aikido is Aikido.

That said, I just spent two very hard training days and at my age there is no way I could keep up that intensity without rest. Nothing stopping me from improving on some other aspect using either an Aikido related exercise (bokken work) or something else. A bit of variety also keeps things interesting.